Experimental Review of Unpolarised Structure Functions

BARYONS 28.10.04 V. Chekelian, Unpolarised SF 1

Experimental Review of Unpolarised Structure Functions

Vladimir Chekelian (MPI for Physics, Munich)

• SF and parton densities • A few historical remarks• Recent JLAB and NuTeV results• HERA results: F2, gluon, s, jets, F2

cc, F2bb, FL ,

NC&CC at high Q2, xF3 ,

pdf• First results from HERA II• Summary


Unpolarised SF and Parton Densities

DIS

Factorisationˆ ( )DIS pdf x

^

perturbative QCD cross section pdf - universal parton distribution functions

QCD evolution (NLO, NNLO)

DGLAP

BFKL, CCFM

2

2

2 2

2

4 3

2

1 (1 ),2 [ ]

e pNC

L Y yy

F F xFd YY

dxd xQ Q Y Y

DIS cross section and SF

22

23

2 1

( , ) ( )

( , ) ( )

2 0

i i i

i i i

L

F x Q x A q q

xF x Q x B q q

F F xF

in QPM:


SF: a few Historical Remarks

1969 SLAC-MIT observation that the proton SF is independent of Q2 at fixed Bjorken x

Bjorken scaling QPM

1974 - … SLAC, EMC, BCDMS, NMC, CDHS, CHARM, CCFR, … logarithmic Q2 dependence of F2(x,Q2) established in e//N scattering experiments

scaling violations QCD


2004 Nobel Prize in Physics for the Discovery of Asymptotic Freedom

David Gross, David Politzer, Frank Wilczek

Selected Publications of Frank Wilczek, with Brief Commentaryhttp://web.mit.edu/physics/facultyandstaff/faculty_documents/wilczek_select_pubs.pdf

QCD: Foundational Papers… in the fifth and sixth papers further experimental consequences, regarding the pointwise evolution of structure functions, were derived. The most dramatic of these, that proton viewed at ever higher resolution would appear more and more as field energy (soft glue), was only clearly verified at HERA twenty years later. …

http://web.mit.edu/physics/facultyandstaff/faculty_documents/wilczek_select_pubs.pdf


Rise of F2 to Low x at HERA

1992

2004

the first HERA data: - discovery of the F2 rise at low x

driven by gluon ! can not rise forever search for new gluon dynamics (saturation effects, …)

after 12 years of HERA: precise data allowing to look for smallest deviations


Kinematic Reach in Q2 and x

fixed target experimentsSLAC,BCDMS,NMC,E665, …

LHC

TeVatron

HERA

full HERA x range is needed for LHC


FT and FL in Resonance Region (JLAB E94-110 )

Quark-hadron duality works well for both FT and FL above Q2 ~ 1.5 GeV2

FT FL

1 < W < 2 GeV


Recent NuTeV Results

- extended to high y (low x)

- most precise measurements of this kind - half of syst.err. of CCFR

-Fe2

2 ( , ) 2 ( )i iF x Q x q q 2

3( , ) 2 ( - )i ixF x Q x q q

Strange Sea Asymmetry vs. the “NuTeV Anomaly”: see talk of S. Forte

F2 xF3


F2(x,Q2) Measurements at HERA

rich possibilities to determine pdfs, test QCD, transition to p, search for saturation effects, …

precision data 2-3% 5 decades in x 5 decades in Q2


Low x at HERAlocally 22( / ln )

QF x 2( )Q

2(22

)2( , ) ( ) QF x Q c Q x from fit

constant at fixed Q2 (x < 0.01) (Q2) depends linearly on ln(Q2) agrees with QCD for Q2 2 GeV2

change of behavior at Q2 1 GeV2

soft pomeron limit = 0.08 (DL)

= 0.08

no taming of the rise of F2 towards low x for Q2 0.5 GeV2


Scaling Violations at Low x

continuous rise towards low x consistent with QCD fit (Q2 3 GeV2) no evidence for new gluon dynamics


Gluon scaling violations pin down the gluon alows to resolve correlation of charm treatment important at Q2 ~ 1 GeV2 the gluon distribution becomes very small xg is NOT an observable Gluon jets, heavy flavours, FL(x,Q2) directly sensitive to xg

H1 inclusive DIS

ZEUS inclusive DIS & jets

sxg

Gluon Density from HERA NLO QCD Fits


Strong Coupling from HERA NLO QCD Fits and Jets

1)((exp)0016.01160.0)(

)((exp)0015.01209.0)(0058.00046.0

2

0048.00049.0

2

HthyM

ZEUSthyM

Zs

Zs

From scaling violations:

Jets

2

( .)

( ) 0.1186 0.0011(exp) 0.005( )s Z

HERA prel

M thy

- small exper. error ~1% - theory error in NNLO expected to be 3 times smaller


Charm Structure Function F2cc (x,Q2)

charm contribution up to 25-30% consistent with with gluon from scaling violations


Beauty Structure Function F2bb (x,Q2)

significance S= /() - track impact parameter in r-

long lifetimes of b,c hadrons• use all tracks with pt>500 MeV and hits in the H1 silicon tracker• jet gives b-direction

F2bb - for the first time

beauty contribution is ~ 3% consistent with with QCD calculations

Contribution to total

b c

u,d,s


Determination of FL(x,Q2) by H1

direct FL measurements still to be done at HERA low beam energy running


NC and CC at High Q2

quarks are pointlike down to proton radius/1000 r < 10-18 m

NC CC at Q2 MZ2,MW

2

unification of electromagneticweak interactions

Neutral Current

e±p

Charged Current high Q2 -> high x

provide possibility to unfold different quark flavours


Structure Function xF3 at HERA reduced NC cross section:

2

2 3NC L

y YF F xFY Y

e-

e+

33 2 2

~ 2/( )

Zv v

e w Z

xFxF u d

a k Q M

xF3 constrains uv,dv at high x


Charged Currents

The CC ep cross section - dominated by d quark

The CC ep cross section - dominated by u quark

CC ep (ep) cross sectionsare suited for constraining d (u) quark density

2 2( , ) ~ ( ) (1 ) ( )pe

CC ux c y d sQ

2 2( , ) ~ ( ) (1- ) ( )peCC x u dc y sQ

][ 32

222 2

42

2

2

21

)(1

2)(

xWYWyWYMQx

MGQdxd

pedL

W

WFCC

),(~ 2QxCC - reduced CC cross section

e-p e+p


PDFs from HERAParton distributions unfolded using HERA NC and CC data only

- H1 and ZEUS parton distributions are in

agreement

-Treatment of systematics, parameterisations

forms and other details are subject to conventions

svv

s

xgdudu

xgAVxgDDUU

,,,

,,,,,,H1

ZEUS

- HERA QCD fits agree with the global fits

Gluon and sea are devided by 20


First Results from HERA II

longitudinally polarised e beam

e beam naturally transversely polarized (Sokolov-Ternov effect)

typically Pe~50% rise-time ~22min

Spin Rotatorsfor the 3 IR’s

detector and luminosity upgrade - efficient data taking since 2003 - O(1fb-1) till 2007

H1

ZEUS

Ee=27.6 GeVEp=920 GeV

s = 319 GeV


CC using Longitudinally Polarised e+

consistent with - linear (1+Pe) dependence - intercept of 0

Linear fit CC= (1+Pe)

( 1) 0.2 1.8( ) 1.6( )totCC eP sta sys pb

Polarisation dependence is firmly established

extrapolation to Pe=-1 tests the absence of right-handed current

after shutdown fall 2004 e- running

( ) (1 ) ( 0)e p e pCC e e CC eP P P

HERA I Pe= 0

HERA IIPe= 33%

HERA IIPe= - 40%


for more than 30 years SF provide a crucial experimental input to establish QPM, QCD and to determine pdfs still very active area: NuTeV, JLAB, … HERA II ~ O(1fb-1) till 2007, low energy running for FL

new level of precision (exp. ~1%, theory NNLO) allows - to investigate applicability domains for different QCD evolutions - to understand high density (low x) QCD - to provide information essential for future LHC collider see HERA-LHC workshop: http://www.desy.de/~heralhc/ large potential for the long term future - HERA III (ed, …), eRHIC, e(ILC)p(HERA,TeVatron)

Summary


Strange Sea Asymmetry Results from NuTeV

the “NuTeV Anomaly”

a 3.1 discrepancy: NuTeV sin2W=0.22770.0016LEP EWWG sin2W=0.22270.0037

dimuon production: s -c -+X (CCFR, NuTeV)

Test one of the interpretations (from many): - strange/anti-strange sea quark asymmetry

Asymmetry S- = xs-dx = x[s(x)-sbar(x)]dx = - 0.0009 0.0014

To explain “NuTeV Anomaly” would require S- = + 0.0060

Experimental Review of Unpolarised Structure Functions

Documents

fixed q2 x

high q2

q2 measurements

independent of q2

unpolarised sflow x

qcd fit q2

unpolarised sfft

unpolarised sfsf